Nonaqueous electrolyte secondary batteries represented by lithium ion batteries are widely used as driving power supplies for portable electronic devices such as mobile phones including smartphones, portable computers, PDAs, and portable music players. Furthermore, the nonaqueous electrolyte secondary batteries are widely used in driving power supplies for electric vehicles (EVs) and hybrid electric vehicles (HEVs and PHEVs) and stationary storage battery systems for applications for suppressing power fluctuations in solar power generation, wind power generation, and the like and peak shift applications for grid power for the purpose of storing power during nighttime and using power during daytime.
In particular, a lithium-cobalt composite oxide (LiCoO2) and different element-doped lithium-cobalt composite oxides doped with Al, Mg, Ti, Zr, or the like have more excellent battery characteristics as compared to others and therefore are widely used. However, cobalt is expensive and the abundance thereof is small as a resource. Therefore, in order to continue to use the lithium-cobalt composite oxide (LiCoO2) and the different element-doped lithium-cobalt composite oxides as positive electrode active materials for nonaqueous electrolyte secondary batteries, the nonaqueous electrolyte secondary batteries are required to have higher performance.
For example, Patent Literature 1 below discloses the invention of a nonaqueous electrolyte secondary battery having high capacity, excellent cycle characteristics, and excellent safety at high temperature. For the purpose of obtaining the nonaqueous electrolyte secondary battery, a mixture of a lithium-cobalt composite oxide and a lithium-nickel-cobalt-manganese composite oxide with a layered structure is used as a positive electrode active material, graphite is used as a negative electrode active material, and a separator with specific physical properties is used in combination therewith. The potential of the positive electrode active material is 4.4 V to 4.6 V on a lithium basis.
On the other hand, in the case of using a negative electrode active material made of a carbon material, lithium can be intercalated only up to the composition of LiC6 and a theoretical capacity of 372 mAh/g is a limit, which is an obstacle to increasing the capacity of batteries. Therefore, the following batteries are under development: nonaqueous electrolyte secondary batteries in which silicon, which is alloyed with lithium, a silicon alloy, or silicon oxide is used as a negative electrode active material with high energy density per mass and volume. For example, silicon allows lithium to be intercalated up to the composition of Li4.4Si, resulting in a theoretical capacity of 4,200 mAh/g, and therefore it can be expected to exhibit much higher capacity than the case of using a carbon material as a negative electrode active material.
For example, Patent Literature 2 below discloses the invention of a nonaqueous electrolyte secondary battery in which one containing graphite and a material containing silicon and oxygen as constituent elements (where the element ratio x of oxygen to silicon satisfies 0.5≦x≦1.5) is used as a negative electrode active material, the proportion of the material containing silicon and oxygen as a constituent element being 3% to 20% by mass on the basis that the sum of graphite and the material containing silicon and oxygen as a constituent element is 100% by mass.
Furthermore, Patent Literature 3 below discloses the invention of a nonaqueous electrolyte secondary battery in which for the purpose of increasing the capacity of the battery by reducing the amount of a negative electrode active material in the case of using lithium nickel oxide, which has high irreversible capacity, as a positive electrode active material, a lithium-nickel composite oxide is used as a positive electrode active material and at least one selected from metallic silicon, a silicon oxide, and a silicon alloy is used as a negative electrode active material.